The continuing objective of this research program has been to characterize at the electron microscopic level various aspects of photoreceptor synaptogenesis in the chick retina. We have localized certain carbohydrate-containing macromolecules on developing photoreceptor synaptic membranes by the use of lectins which bind to certain sugar residues and have studied how they become more or less accessible to the EM labeling technique during synapse formation. We now want to study the nature of the protein moiety of these glycoprotein carbohydrates by the use of polyacrylamide gel electrophoresis in combination with specific iodinated lectin staining techniques. We also want to extract these developmentally regulated glycoproteins from the gels and by the use of immunocytochemical techniques localize the individual glycoproteins in tissue sections. Further studies will be carried out to characterize by EM and freeze-fracture developmental changes of postsynaptic membranes at photoreceptor synapses and to correlate these changes with photoreceptor presynaptic development as described in our previous thin section and freeze-fracture studies. Particular emphasis will be placed on several new postsynaptic relationships which we have described recently and which may be unique to the avian retina. Finally, cholera toxinperoxidase labeling techniques will be used to localize by EM other sugar containing macromolecules (gangliosides) on developing photoreceptor synpatic membranes and this data will be compared with the glycoprotein localization observed by immunocytochemical techniques.